When you have the file highlighted on your desktop or in Finder, either choose "Get Info" from the File menu or hit Command-I to bring up the Info box. In the "Open with:" section, it probably has Photoshop selected. Change it to Preview, and then click the "Change All..." button and confirm your selection. I do this periodically, and if I need to open a file in Photoshop, I simply drag it to the Photoshop icon in my Dock.